Prior art methods of binary digital transmission employed either synchronous or asynchronous transmission, without the ability to change between these modes due to the synchronising characters by which the former is structured, and the start and stop bits employed in the latter, framing each character transmitted for character synchronisation. The use of a unique synchronising character in synchronous data transmission limits the rate of data transmission, due to the requirement that transmitted characters be continuously checked and if necessary modified to preserve the uniqueness of the synchronising character.